The present invention relates generally to electric power conversion systems, and more specifically to a means for reducing non-monotonic increases in output signals encountered in high-voltage applications of the Milberger converter.
In conventional high-voltage, high-frequency converter transformers, great effort is expended in trying to reduce the leakage inductance in the windings, and keep stray capacitance to a minimum. The reason is that the ideal output of such systems is a full wave rectified square wave which has no undesirable gaps. Stray capacitance and inductance leakage in the transformer windings are known to produce undesirable resonance signals which degrade the waveshape of the output and causes a voltage ripple.
The task of eliminating non-linearities from output signals from high-voltage applications of the Milberger converter is alleviated, to some extent, by the following U.S. Patents, the disclosure of which are incorporated by reference:
U.S. Pat. No. 3,868,561 issued to Matthes; PA1 U.S. Pat. No. 3,340,458 issued to Keller; PA1 U.S. Pat. No. 4,341,990 issued to David; and PA1 U.S. Pat. No. 3,925,715 issued to Venable.
The Matthes patent discloses a resonant circuit transformer arrangement including a ripple smoothing filter formed from a transformer used to couple a rectifier to an inverter.
The Davis patent discloses a circuit for reducing high frequency line ripple in regulating circuits.
The Keller patent discloses an "LC" filter choke using two transformers each 180.degree. out of phase which cancel ripple.
The Venable reference discloses a pulsewidth modulated DC to DC converter with output circuits which replace conventional output inductive filters.
The references cited above are exemplary in the art and depict the use of output circuits, including the use of a second transformer in the Keller reference, to condition converter output signals. However, close observation of the high-voltage application of the Milberger converter has indicated that the undesirable non-monotonic degradation of the output waveform is a product of a resonance frequency signal produced by inductances and capacitances particularly in the secondary windings. This suggests that rather than the "add-on" solutions of the prior art references including L-C filters, etc., perhaps the characteristics of the secondaries themselves should be modified.
The Milberger converter is best understood by referring to the U.S. patent application Ser. No. 032,406 entitled "The Milberger Converter", filed on Mar. 27, 1987, the disclosure of which is incorporated by reference. The Milberger converter has an advantage over conventional converters in that its output is conditioned by two independent square waves which either add or cancel. The output voltage is directly proportional to a percentage of addition time to the total time, i.e., Vo equals Vp (T.sub.ADD /T.sub.TOTAL).
As disclosed in the above-cited reference, the Milberger converter's 100 percent dynamic range, small size, and reduced number of parts are among its main advantages. However, a problem has been encountered when the Milberger converter is used in high voltage and high power applications. Medium power is, in the present context, defined as electrical power of around 100 watts, and high power is considered to be electrical power above 10 kw. In high voltage and high power applications there exists a non-monotonic increase in the output voltage of the Milberger converter, which occurs when its control circuit commands a linear increase. Investigation of the phenomenon indicates that it is caused by the occurrence of the presence of an ultra-high frequency ripple on top of the pulses of the output waveform. When this ripple is in phase with the output signal, it adds and the output increases. As the phase of the ripple shifts, it alternately increases and decreases on top of the output signal.
The task of reducing these non-monotonic increases in the output signals of Milberger converters in high power applications is alleviated by the U.S. patent application entitled "High-Voltage Milberger Slip Slide Power Conditioner" by F. B. Jones et al, the disclosure of which is incorporated by reference. Ihe disclosure of Jones et al describes the design of a slip slide power conditioner in which the output signal of the Milberger conditioner is conditioned by a signal processing circuit to remove non-linearities due to secondary ringing or resonance that beats with harmonics of the chopper frequency.
The system proposed by Jones et al is an effective solution which conditions the output signals of the Milberger converter in high power applications. Jones et al follow the tradition of the above-cited references of Venable, Davis, Keller, and Matthes by presenting an "add-on" circuit to condition the output of a transforxer or a converter. However, a closer observation of the origin of the present problem indicates the following. The ringing frequency is a product of the output transformer inner winding capacitance and inductance leakage. Since the origin of the problem occurs in the output transformer, there exists the need for a more direct solution to the present problem: change the characteristics of the output transformer. The present invention satisfies that need.